Role of colloids in actinide migration, in different geological conditions

The role of colloids in actinide subsurface migration was studied in different geochemical conditions (pH, Eh, chemical composition) of two nuclear waste sites of Russia. It was established that in oxidic conditions U and Np were present in hexa- and pentavalent forms with low affinity of sorption onto natural aquatic colloids. Plutonium was present as a mixture of valence states (IV-VI) and was mostly bound to colloids. The colloids were characterized by TEM with EDX and SAED and preferential binding of actinides to different colloidal phases was studied by nano-SIMS. In anoxic conditions uranium is found as U(IV) and forms intrinsic (or Eigen-colloids) that sorb or co-precipitate other actinides. The data on preferential sorption of actinides onto different colloids was supported by sorption experiments in well defined laboratory conditions. Actinide speciation was studied by XPS and XAFS as well as solvent extraction. (authors)